In the area of mobile data communication systems, the design of the receiver is a crucial factor in the determination of the performance of such systems. Input to the receiver is an RF signal from the antenna as depicted in FIG. 1. The output of the receiver is the data which ideally is the same as the transmitted data.
Mobile radio systems are expected to operate in a variety of environments. It is therefore conceivable that a mobile radio will operate in a static environment in which the primary impairment is the effect of receiver front end noise, which may be large relative to the signal if the radio is used within a building and subject to path loss through the walls of the building. Other mobile radios operating on the same system may be operating in a mobile environment in which the primary sources of degradation are fading and multipath. The best receiver technique for the static environment is not necessarily the best for the mobile environment.
Data sent by such transmitters is often carried by the phase of the carrier signal, especially in commonly used modulation techniques such as BPSK, QPSK, OQPSK, .pi./4 shifted DQPSK, MSK, tamed-FM, LRC, or GMSK. To recover data correctly, the receiver must therefore accurately detect the phase of the data.
In a stationary environment inside a building, received signals are primarily degraded by additive white Gaussian noise (AWGN) caused by the receiver front end. The signal received by either the base station or the portable unit may be very weak because of path loss resulting from its propagation through the building walls, and as a result, the ratio of signal to noise power (signal-to-noise ratio or SNR) may be quite low. The performance of the receiver is a strong function of the SNR. If the portable unit is stationary, the effects of Rayleigh fading and multipath on the receiver will be relatively minor. It is well known that for such a channel, optimum performance is obtained using a coherent demodulation technique in which the demodulation uses a frequency source that phase locks to the carrier frequency of the received signal.
However, when the portable unit is moving, and especially when the motion is rapid, the signal is degraded by Rayleigh fading and multipath in addition to AWGN. The Rayleigh fading causes rapid phase changes to the received signal. In these cases, non-coherent demodulation may prove to be superior to using coherent techniques for phase recovery. Non-coherent techniques may include the use of limiter discriminators or differential techniques in which the receiver can make decisions based on phase changes as opposed to the absolute phase of the received signal. It is common practice in the industry to use either one of the two above demodulation techniques for data recovery in receivers for mobile communications. These receivers do not adapt to the different environments in which the radios operate. The problem is then presented to design a receiver that takes into account these sources of degradation which are variable in nature due to the changes in the environment of a mobile radio.